Underground gasification of coal deposits in the form of thin seams located at great depth involves a number of problems.
For economic reasons, it is necessary to develop large gasifiers. In the present state of the art, this implies that the gasifiers be developed from long in-seam holes.
In order to resist rock pressure, the wells must be coated solidly; the coating must not be subjected simultaneously to high temperatures and stresses resulting from the high lithostatic pressure. This requirement can be met by using conventional metallic casings if the retreating system is adopted, in which the wells are used all the time to inject gasifying agents at low temperature.
The arrangements must also ensure an intimate contact between gasifying agents and coal; this condition is essential to produce good quality gas.
U.K. Pat. No. 2004297 A describes a retreating gas-recovery method, in which close contact between gasifying agent and coal is achieved by a methodical stowing (filling) of already gasified zones, the filling material being a granulated material transported pneumatically through the wells used for gasifying agent injection.
Taking into account the large void subsisting after coal gasification, this process requires the injection of very large quantities of material and filling may prove to be very expensive.
U.S. Pat. No. 4,334,579 describes a retreating method of gas recovery, in which close contact between gasifying agent and coal is achieved without filling by effecting periodically a controlled retraction of the gasifying agent injection point so as to permanently keep a large enough quantity of coal between the gasifying agent injection point and the already gasified zones.
In one variant of this process, the gasifying agents are injected into long in-seam wells, the injection point being gradually retracted from the well end to its starting point, using a retractable or thermodegradable injection tube.